hugolib: Extract date and slug from filename

This commit adds `enableFilenameDateAsFallback` which, when enabled and no date is set in front matter, will make Hugo try to parse the date from the content filename.

Also, the filenames in these cases will make for very poor permalinks, so we will also use the remaining part as the page `slug` if that value is not set in front matter.

This should make it easier to move content from Jekyll to Hugo.

Fixes gohugoio#285
Closes gohugoio#3310

hugolib/page.go

@@ -1373,6 +1373,30 @@ func (p *Page) update(frontmatter map[string]interface{}) error {
 	return nil
 }
 
+// A Zero date is a signal that the name can not be parsed.
+// This follows the format as outlined in Jekyll, https://jekyllrb.com/docs/posts/:
+// "Where YEAR is a four-digit number, MONTH and DAY are both two-digit numbers"
+func dateAndSlugFromBaseFilename(name string) (time.Time, string) {
+	withoutExt, _ := helpers.FileAndExt(name)
+
+	if len(withoutExt) < 10 {
+		// This can not be a date.
+		return time.Time{}, ""
+	}
+
+	// Note: Hugo currently have no custom timezone support.
+	// We will have to revisit this when that is in place.
+	d, err := time.Parse("2006-01-02", withoutExt[:10])
+	if err != nil {
+		return time.Time{}, ""
+	}
+
+	// Be a little lenient with the format here.
+	slug := strings.Trim(withoutExt[10:], " -_")
+
+	return d, slug
+}
+
 func (p *Page) GetParam(key string) interface{} {
 	return p.getParam(key, false)
 }

hugolib/page_test.go

@@ -1873,6 +1873,43 @@ tags:
 	}
 }
 
+func TestDateAndSlugFromBaseFilename(t *testing.T) {
+	t.Parallel()
+
+	assert := require.New(t)
+
+	tests := []struct {
+		name string
+		date string
+		slug string
+	}{
+		{"page.md", "0001-01-01", ""},
+		{"2012-09-12-page.md", "2012-09-12", "page"},
+		{"2018-02-28-page.md", "2018-02-28", "page"},
+		{"2018-02-28_page.md", "2018-02-28", "page"},
+		{"2018-02-28 page.md", "2018-02-28", "page"},
+		{"2018-02-28page.md", "2018-02-28", "page"},
+		{"2018-02-28-.md", "2018-02-28", ""},
+		{"2018-02-28-.md", "2018-02-28", ""},
+		{"2018-02-28.md", "2018-02-28", ""},
+		{"2018-02-28-page", "2018-02-28", "page"},
+		{"2012-9-12-page.md", "0001-01-01", ""},
+		{"asdfasdf.md", "0001-01-01", ""},
+	}
+
+	for i, test := range tests {
+		expectedDate, err := time.Parse("2006-01-02", test.date)
+		assert.NoError(err)
+
+		errMsg := fmt.Sprintf("Test %d", i)
+		gotDate, gotSlug := dateAndSlugFromBaseFilename(test.name)
+
+		assert.Equal(expectedDate, gotDate, errMsg)
+		assert.Equal(test.slug, gotSlug, errMsg)
+
+	}
+}
+
 func BenchmarkParsePage(b *testing.B) {
 	s := newTestSite(b)
 	f, _ := os.Open("testdata/redis.cn.md")